High voltage transformers convert voltages from one level or phase configuration to another, usually from a lower to a higher voltage. High voltage transformers can be configured to provide for electrical isolation and may employ features for power distribution, control, and instrumentation applications. High voltage transformers usually function based on the principle of magnetic induction between coils to convert voltage and/or current levels. An alternating current in a primary winding associated with the transformer can create a changing magnetic field, which in turn induces a variable voltage in a secondary winding. Note that high voltage transformers can be configured in the context of either a single-phase primary configuration or a three-phase configuration.
One of the many non-ideal characteristics of a real transformer is leakage current associated with the transformer. In general, leakage current can be defined as a capacitive current that flows as a result of a capacitive coupling between the secondary and primary windings associated with the transformer. The high voltage present on the upper end of the secondary winding may be responsible for a relatively large capacitive current that flows from the secondary winding and then back to the primary winding (and sometimes to a core).
Transformer designs can be manipulated to minimize such leakage current; however, the leakage current may not be completely eliminated. The leakage current is not always a problem, particularly if the leakage current is capable of being minimized in the transformer design. Such leakage current may, however, become a problem with an AC (Alternating Current) output high voltage power supply.
An AC output high voltage power supply utilizes a secondary low-side current sensing approach in which the output current is returned to the secondary winding through a current sense resistor. However, the current sense resistor senses both the output current and the leakage current returning to the secondary winding. Such an approach typically generates a resulting error in a current monitor, which may present a significant problem. The measurement of the return current, however, can be filtered to eliminate the AC leakage current in DC (Direct Current) outputs.
FIG. 1 illustrates a circuit diagram of a prior art high voltage power supply 100 associated with a high voltage transformer 150. The high voltage power supply 100 generally includes a high voltage transformer 150 comprising a primary winding 120 and a secondary winding 130, which are provided with respect to a core 110 of a soft-magnetic material. The high voltage power supply 100 further includes a power stage that may include, but is not limited to, two operational amplifiers 140 and 160 driven 180 degrees out of phase, which are capable of switching the voltage across the transformer primary winding 120, thereby producing the desired secondary voltage Vout. The secondary current can then be measured utilizing a small sense resistor RCS located between a ground terminal and the low side of the secondary winding 130. The voltage across the small sense resistor RCS can be rectified and scaled and utilized for diagnostics and fault detection purposes.
The bold arrows 163 and 165 depicted in FIG. 1 generally indicate the path of conventional (ideal) primary current flow in the high voltage transformer 150. Note that the direction of primary current flow indicated in FIG. 1 is depicted for general illustrative purposes only; however, it can be appreciated that the primary current may change direction periodically. Note that the arrows 157, 159, and 161 depicted in FIG. 1 can define a path for the leakage current, starting at the high voltage side of the secondary winding 130. The leakage current represented by arrows 157, 159 and 161 may constitute an alternating current and is capable of changing direction at a particular switching frequency.
The leakage current represented by arrows 157, 159, and 161 in FIG. 1 generally flows from the high voltage side of the secondary winding 130 via the leakage capacitors CL1 and CL2 (which are not true capacitor components, but rather represent the parasitic capacitance between the high voltage secondary winding and the low voltage primary winding), the amplifier circuits 140 and 160 to a ground terminal and back to the secondary lower side through the current sense resistor RCS. The primary current 165 flows through the primary winding 120 during operation. Hence, it is extremely difficult to differentiate between the primary current represented by arrows 163 and 165 and the leakage current represented by arrows 157, 159, 161, where the primary current indicated by arrows 163 and 165 is many orders of magnitude higher than the leakage current.
The majority of prior art approaches for compensating leakage currents in current monitors can be derived from a secondary side current measurement. Such an approach can post-scale rectified and filtered current sense signals by a fixed amount to compensate for an average leakage current. The leakage current is always purely capacitive whereas a load current can be reactive or resistive or a combination, hence, such prior art compensation methods can only function over a limited range of load currents. Additionally, an average leakage current can be determined only if the actual transformer primary to secondary capacitance is nearly equal to the reference capacitance. Finally, the idealized leakage capacitance is not necessarily constant, but may vary with voltage and, to some extent, environmental conditions.
Based on the foregoing, it is believed that a need exists for an improved method capable of compensating for leakage current in a high voltage transformer. A need also exists for an improved means of sensing leakage current, as described in greater deal herein.